1. Field of the Invention
This invention relates to a magnetic device, a magnetic recording head, and a magnetic recording apparatus of the microwave assisted type suitable for realizing data storage with high recording density, high recording capacity, and high data transfer rate.
2. Background Art
In the 1990s, the practical application of MR (magnetoresistive effect) heads and GMR (giant magnetoresistive effect) heads triggered a dramatic increase in the recording density and recording capacity of HDD (hard disk drive). However, in the early 2000s, the problem of thermal fluctuations in magnetic recording media became manifest, and hence the increase of recording density temporarily slowed down. Nevertheless, perpendicular magnetic recording, which is in principle more advantageous to high-density recording than longitudinal magnetic recording, was put into practical use in 2005. It serves as an engine for the increase of HDD recording density, which exhibits an annual growth rate of approximately 40% these days.
Furthermore, the latest demonstration experiments have achieved a recording density exceeding 400 Gbits/inch2. If the development continues steadily, the recording density is expected to reach 1 Tbits/inch2 around 2012. However, it is considered that such a high recording density is not easy to achieve even by using perpendicular magnetic recording because the problem of thermal fluctuations becomes manifest again.
As recording schemes possibly solving the above problem, the “patterned medium recording” and the “thermally assisted magnetic recording” are proposed. Both schemes are currently under active research and development in Japan and abroad. In the case of the patterned medium recording, it is an urgent need to commercialize a medium manufacturing process technology for cost-effectively manufacturing a fine pattern of isolated bits measuring 20 nanometers or less with precision on the order of nanometers to subnanometers.
On the other hand, in the case of the thermally assisted magnetic recording by light irradiation, it is essential to commercialize a hybrid magnetic head in which a near-field optical device for instantaneously heating a microscopic area of the medium to decrease its coercivity is placed close to a recording magnetic pole for applying a recording magnetic field to the area of decreased coercivity. Furthermore, in the thermally assisted magnetic recording, it is also important to develop a recording magnetic material having a very high magnetic anisotropy energy (Ku), which cannot be written by the conventional magnetic heads.
In contrast, as a recording scheme different from the thermally assisted magnetic recording, the “microwave assisted magnetic recording” is proposed (e.g., U.S. Pat. No. 6,011,664). In this technique, a radio-frequency magnetic field having a frequency sufficiently higher than the recording signal frequency is applied to a prescribed microscopic site of the magnetic recording medium to decrease the coercivity of the site in the recording signal frequency region from its original coercivity Hc1 to the half or less, Hc2. Application of a recording magnetic field to this site at the time of decreasing the coercivity enables magnetic recording to a magnetic medium having a higher magnetic anisotropy energy (Ku) and a higher-density recording potential.
As a method for applying a radio-frequency magnetic field, U.S. Pat. No. 6,011,664 discloses passing a radio-frequency current through a coil coupled to a magnetic pole to excite the magnetic pole and applying the radio-frequency magnetic field generated from the magnetic pole to the magnetic recording medium. However, in this method, as the medium recording site is downsized to increase the recording density, the strength of the radio-frequency magnetic field applicable to the site sharply decreases. Hence, unfortunately, it is difficult to decrease the coercivity of the recording site, that is, to achieve microwave assisted magnetic recording.
To solve this problem, a technique of using a spin oscillator as an oscillation source for a radio-frequency magnetic field is proposed (US Patent Application Publication No. 2005/0023938, and US Patent Application Publication No. 2005/0219771).
US Patent Application Publication No. 2005/0023938 and US Patent Application Publication No. 2005/0219771 disclose a method for using a spin oscillator as a source for generating a radio-frequency magnetic field. Upon passage of a DC current, the spin of electrons passing through a spin polarization layer is polarized. A spin oscillation layer receives a spin torque by the polarized electron flow, and its magnetization undergoes ferromagnetic resonance. Consequently, the spin oscillator, which is composed of the spin oscillation layer laminated with the spin polarization layer via a nonmagnetic layer, generates a radio-frequency magnetic field from the spin oscillation layer.
This phenomenon prominently emerges for the device size of several ten nanometers or less. Hence the range of the radio-frequency magnetic field generated from the device is limited to within a microscopic region of several ten nanometers or less from the device. The oscillation frequency is set equal to or near the ferromagnetic resonance frequency of the recording layer of the magnetic recording medium, and the magnetic recording head with the spin oscillator placed near the recording magnetic pole is closely opposed to the magnetic recording medium. Then the radio-frequency magnetic field generated from the spin oscillator can be applied only to the microscopic recording site of the medium recording layer. Consequently, it is possible to decrease only the coercivity of the microscopic recording site.
At this time of decreasing the coercivity, a recording magnetic field is applied to this recording site using the recording magnetic pole. This enables magnetization reversal of only the recording site, that is, writing of information.
Furthermore, the power consumption of the spin oscillator is as low as that of the conventional GMR devices and TMR (tunneling magnetoresistive effect) devices, and the heat generation is also negligible. Moreover, the head structure with the spin oscillator placed close to the recording magnetic pole can be fabricated in a manufacturing process similar to that for the conventional magnetic heads. Thus the manufacturing cost is also very inexpensive. Hence the microwave assisted magnetic recording based on the spin oscillator is promising as a future magnetic recording scheme.